A diverter switch included in a tap changer is usually used in connection with a transformer to enable tapping at different voltage levels. This occurs in cooperation with a selector connected to the diverter switch. When the power output from a transformer is to be changed from one voltage level to another, this occurs by first connecting the selector to that tapping point of the transformer winding which corresponds to the new voltage level while the diverter switch is still feeding from the existing voltage level. The connection of the selector thus takes place without current load. When the selector is connected to the tap for the new voltage level, a switching operation then takes with the aid of the diverter switch such that output current is taken out from the new tapping point of the transformer. When a transformer has a plurality of tapping points, switching normally only occurs between two tapping points which are close to each other in terms of voltage. If an adjustment to a more distant location should be required, this takes place step by step. A diverter switch of the kind referred to here is normally used for control of power or distribution transformers. The invention is not, of course, limited to this type of application but may also advantageously be used for control of other types of power transmission or distribution products, such as reactors, phase shifters, capacitors or the like.
The operation of the diverter switch involves commutation from one circuit to another with en ensuing occurrence of an electric arc. To avoid polluting the insulating medium, such as oil, into which the diverter switch is normally immersed, and to reduce the wear of the switch contacts, it is previously known to use vacuum switches for those switching operations where an arc arises. The electrical contact wear will then only arise in the vacuum switch. For an appropriate procedure from an electrical point of view, a diverter switch of this kind is provided with at least one main branch and one resistance branch.
A diverter switch of the above kind is previously known from, for example, U.S. Pat. No. 5,786,552. The diverter switch described therein thus has one main branch and one resistance branch, in the steady state connected in parallel and connected to an output line. Each branch is provided with a vacuum switch and a contact connected in series therewith. These are operated in a definite sequence when diverter switching is to take place, in which case it is important to ensure that the main branch is operated before the resistance branch. In this way, the vacuum switch of the main branch may be dimensioned for breaking of the load current only and the vacuum switch of the resistance branch for the circulating current that arises. In case of the reverse sequence, the vacuum switch of the main branch would be forced to break the sum of these currents and thus be dimensioned therefor. Each contact is operated in different directions in a reciprocating movement to bring about an operating sequence where the main contact is operated before the resistance contact. For this reason, the contact system requires special arrangements, which implies a complex mechanical solution to the diverter switch, which thus renders difficult an efficient adaptation of the production because of the relatively complicated installation of the diverter switch. In addition, this solution to diverter switches is relatively space-demanding.
Additional examples of similar devices are described, for example, in WO94/02955, WO99/60588, WO00/24013, WO02/31846, EP 712140, EP 650637, EP 1197977, GB 2000911, U.S. Pat. No. 4,978,815, DE 29622685 and DE 4315060.
The object of the present invention is to provide a diverter switch and a method for operating such a switch, wherein said disadvantages of the prior art are eliminated, and thus achieve an operation wherein it is ensured in a simple manner that the main contact is always operated before the resistance contact.
The components for a diverter switch of this kind are dimensioned, inter alia, for transmitting a highest load current in continuous operation. However, it may be desired to utilize these components also for a higher load current. One known way to achieve this is to provide a diverter switch with a bypass function, which implies that the load current is substantially passed via a bypass connection during continuous operation. One advantage is that the load current may be increased since vacuum switches and contacts are loaded substantially only instantaneously during the switching operation. A side effect is that a bypass makes possible reduction of the losses in a diverter switch. Further, the losses in the diverter switch may then be reduced. A disadvantage of known bypass functions of this kind is that the diverter switch must be provided with complicated and expensive means for operation of the additional components.
Further objects of the present invention are therefore to provide a diverter switch with a bypass function and a method for operating such a switch, still achieving a fast, simple and reliable operation of the bypass function.